Head-up display devices, whether they are worn or not, in particular allow a conformal “symbology” to be displayed on the outside world, i.e. a set of symbols the position of which in front of the eye of the pilot allows a superposition with the corresponding elements in the outside world. It may for example be a question of a speed vector, of a target on the ground or in the air, of a synthetic representation of the terrain or even of a sensor image.
This conformal display requires knowledge of the position and attitude of the aircraft and, for display devices worn on the head, the attitude of the display with respect to a fixed coordinate system linked to the aircraft. These various positions and attitudes are delivered, with respect to the aircraft, by avionic systems, and, with respect to those of the display, by the DDP posture-detecting device.
For example and in particular, the avionic systems used to deliver the position and attitude of an aircraft may respectively be:    a global positioning system (GPS), and    an inertial reference system (IRS) or an attitude and heading reference system (AHRS).
As is known, a harmonization is carried out on installation of the head-up display system, whether it be worn or not, in a cockpit, in order to calculate the angular corrections that must be made to pass from the display coordinate system to the aircraft coordinate system, and in order to obtain a conformal head-up display.
However, at the present time, certain worn head-up display devices exhibit a certain mobility between the display device or display and the worn portion of the DDP posture-detecting system, because of a mechanical non-rigidity between these two elements, i.e. the display and the worn mobile portion of the DDP, for example when there is a device for toggling the display alone out of the field of view of the operator. It is then necessary, when the display is once again toggled back into the field of view of the operator, to carry out a new harmonization in order to calculate new angular corrections to be made to the head once the head-up display is installed, and thus to make it possible to display a conformal symbology on the head-worn display device.
In order to allow and facilitate this need for relatively frequent re-harmonization, it is common to install, on board the aircraft, a dedicated instrument, called the BRU (boresight reference unit or boresight reticle unit).
The BRU, installed in the cockpit facing the head of the operator, displays a collimated symbol with a fixed orientation that is known to the head-up system.
Each time the conformal symbology must be readjusted, i.e. on each re-harmonization, the operator aligns a symbol displayed on the head-up display with the collimated symbol of the BRU.
When the symbol displayed on the head-up display, i.e. the display, is aligned with the collimated symbol, the system for harmonizing the output of the detecting device then calculates a rotation matrix from three correction angles, in order to re-harmonize the attitude of the coordinate system of the display with respect to the coordinate system of the aircraft.
The main drawback of this harmonization system based on the use of a BRU is the need to carry an additional piece of equipment dedicated to this single readjustment or harmonization function, inducing a cost in terms of installation complexity and an additional bulk and weight that may be unacceptable, in particular for small civilian aircraft. This BRU must be supplied with power through robustly installed electrical cabling. This BRU requires a harmonization procedure during its installation and introduces an additional source of error, called the harmonization error of the BRU. A risk of dealignment by movement is also possible, for example when the pilot enters or exits the cockpit or during a maintenance operation.
Furthermore, the exact orientation parameters of this BRU on the carrier, i.e. the carrying structure of the aircraft, must also be introduced into the HMD display system, and the BRU must therefore always remain perfectly fixed with respect to the carrier. However, current mechanical technologies do not allow the BRU to be mounted in the cockpit in such a way as to guarantee a zero risk of variation over time. Specifically, the vibrational environment and actions of the pilot and of maintenance operators in particular may cause slight rotations or movements of the BRU, these creating a line-of-sight error that is uncompensatable and in many cases undetectable, and therefore preventing any subsequent re-harmonization.
A first technical problem is that of providing a readjusting system and method that allows the symbology to be readjusted with the outside world when the HWD/HMD head-up display system comprises a mechanism for disengaging and re-engaging the display in the field of view of the pilot (source of misalignment) and that avoids the need to use a calibration marker installed in the cockpit and also a source of error.
A second technical problem is that of determining, with greater precision, the relative orientation between the display D0 and the device that is tightly fastened to the head/the mobile portion of the subsystem for detecting the posture of the head when the HWD/HMD head-up display system comprises a mechanism for disengaging and re-engaging the display in the field of view of the pilot.
A third technical problem is to correct the orientation of the aircraft with respect to the Earth delivered by the inertial measurement unit of the aircraft, and in particular the delivered heading, the value of which is generally not known with sufficient precision for a conformal display when this unit is an AHRS.